Composite materials have well-documented advantages over traditional construction materials, particularly in providing excellent mechanical properties at very low material densities. As a result, the use of such materials is becoming increasingly widespread and their fields of application range from “industrial” and “sports and leisure” to high performance aerospace components.
Prepregs, comprising a fibre arrangement impregnated with resin such as epoxy resin, are widely used in the generation of such composite materials. Typically a number of plies of such prepregs are “laid-up” as desired and the resulting laminate is cured, typically by exposure to elevated temperatures, to produce a cured composite laminate.
In a typical lay-up procedure, the prepreg is provided as a roll of material to provide length (e.g. of the order of several meters) of prepreg of a predefined width (e.g. of the order of several centimeters) and thickness (e.g. of the order tenths of millimeters up to several millimeters). Typically cure ply thicknesses of prepregs for aerospace applications range from 0.125 mm to 0.25 mm and their fibre average weights range from 100 to 300 g/m2.
To facilitate such unrolling, the prepreg typically has an outer layer of backing paper which prevents the adjacent layers of prepreg from sticking together. Such backing paper is discarded before lay-up.
A common method on laying up such prepreg is by means of an automated lay-up apparatus. This is a much more efficient method of laying up prepreg as compared to conventional hand lay-up. However, it does impose additional constraints on the dimensions of the prepreg, if it is to automatically lay down the prepreg at an acceptable quality standard.
When laying prepreg to form a structure which has relatively high curvature, it is known to lay down strips of prepreg which have a much smaller width than conventional prepreg. This reduces any wrinkling that a wider strip of prepreg may suffer from during lay-up.
It is known in the art to produce so-called slit tape prepreg, which is produced by passing a sheet of prepreg through a slitting or cutting unit to produce a plurality of parallel strips of prepreg. The width of the strips produced are very tightly controlled and can be specified to within a fraction of a millimeter.
Sheets of prepreg are typically manufactured with outer sheets of backing paper which allows the high compressive forces required for resin impregnation to be applied. As discussed above, the backing paper also allows the prepreg to be rolled up onto itself once produced without adjacent layers of prepreg adhering to each other.
However, it is known that passing prepreg with its backing paper through the slitting unit comprising slitting blades produces paper debris which produces an unacceptable result. It is therefore common practice to remove the backing paper before slitting.
Once the strips of prepreg are produced it is conventional to apply a backing sheet (which is wider than the strip of prepreg) and way-wind the strip onto a bobbin or spool. Such a bobbin is usually capable of holding several thousands of meters of such strip prepreg.
Such a bobbin or a plurality thereof, can be adapted for use with an automated lay-up apparatus, which automatically unravels the tape, removes the backing sheet and lays down the strips of prepreg. Typically a plurality of strips of prepreg are laid down parallel to each other.
Clearly it is desirable that any gaps or overlaps between adjacent strips are minimised. This is all the more important when a high quality result is essential, such as when constructing an aerospace component.
However it has been found that an intentional gap must be imposed if overlap of adjacent tapes is to be avoided. This is because variation in the width of the strips of prepreg has been observed in practice, and overlapping strips are more problematic than a gap between strips.
The present invention aims to mitigate or at least obviate the above described problems and/or to provide advantages generally.